(1) Field of the Invention
This invention relates to an image forming apparatus with a toner density control device, especially to the one for forming an image using the electrophotography system and a dual component developer.
(2) Description of the Related Art
A dual component developer comprises a toner for visualizing an electrostatic latent image on a photoconductive drum and a carrier for charging the toner by friction and carrying the toner to a developing area. While the carrier is kept in a certain amount in a developing device, the toner is consumed by the image forming.
A toner density, which would be reduced by the consumption of the toner and thus affect an image density, is maintained at a certain level by additional supply of the toner. The reduction in the toner density is detected magnetically or optically, and the toner is supplied based on the detection result.
In a detecting method using magnetism, the change in the amount of the carrier existing in a certain volume (including a space among toner and carrier grains) of the developer is detected in the form of the change in permeability.
A detecting method using optics is generally used for a color toner. The developer is illuminated by a light, and the toner density is detected in the form of the reflectance of the light.
The detected permeability or reflectance is converted into a voltage, which is compared with a reference voltage. The necessary amount of toner to be supplied is obtained based on the comparison result.
In the method using magnetism, the toner density is not directly detected but through the amount of the carrier. Accordingly, the detected toner density is influenced by elements other than the consumption of the toner, such as apparent density (weight/volume of the developer) or fluidity of the toner. In the method using optics also, the other elements influence the detected toner density as will be described below.
The followings are the main elements which influence the toner density detected by the above two methods.
1) A toner is usually added with approx. 0.2 to 1% of silica diffused therein as an after-treatment for improving fluidity and stabilizing chargeability. As the toner is stirred more and more, the silica is gradually separated from the toner. Accordingly, more use of the developer causes a reduction in the toner fluidity. As a result, the detected toner density tends to be higher than the actual one.
2) When consumed quickly, the toner is stirred for a short time, and so the silica is separated in a small amount. It means the difference between the detected density and the actual density depends on how quickly the toner is consumed.
3) In the case of the reversal development, in which a toner is adhered on a non-charged area of an electrostatic latent image, the toner is consumed from a portion thereof having a low chargeability. As more and more copies are made, highly chargeable portions of the toner are accumulated. Accordingly, the toner tends to cohere to the carrier. Such a phenomenon causes a change in the apparent density and a reduction in the toner fluidity. As a result, the detected density tends to be higher than the actual one.
4) Especially in the method using optics, there occurs the spent phenomenon of the carrier. In other words, as the developer is used more and more, the carrier is adhered with a broken toner and thus reflects the light although the carrier would normally absorb the light. This phenomenon also causes the detected toner density to be higher than the actual one.
As mentioned in 1) through 4), the detected density becomes higher and higher than the actual one as the developer is used more and more. Accordingly, the image density gets lower and lower.
Japanese Patent Laid-Open No. 57-73771 discloses revising the reference voltage with consideration that an accumulative number of operation times of the developing device (namely, an accumulative number of copies which have been made; will be referred to as the copy number) influences the apparent density. This disclosure has the following problems.
1) Since the toner consumption per copy is not always the same, a toner consumption which is assumed from the copy number is quite different from an actual one. This leads to an inappropriate revision of the reference voltage.
2) The disclosure does not consider that the apparent density is influenced by the environment of the developing device (for instance, humidity), which makes a difference between the detected toner density and the actual one.